Method of controlling a brushless permanent-magnet motor

ABSTRACT

A method of controlling a brushless permanent-magnet motor. The method includes commutating a winding of the motor at times that are retarded relative to zero-crossings of back EMF in the winding.

REFERENCE TO RELATED APPLICATION

This application claims priority of United Kingdom Application Nos.1310574.7 and 1310572.1, filed Jun. 13, 2013, the entire contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of controlling a brushlesspermanent-magnet motor.

BACKGROUND OF THE INVENTION

There is a growing need to improve the efficiency of brushlesspermanent-magnet motors.

SUMMARY OF THE INVENTION

The present invention provides a method of controlling a brushlesspermanent-magnet motor, the method comprising commutating a winding ofthe motor at times that are retarded relative to zero-crossings of backEMF in the winding when operating at speeds greater than 50 krpm.

At speeds greater than 50 krpm, the length of each electrical half-cycleis relatively short and the magnitude of the back EMF is relativelylarge. Both of these factors would suggest that advanced commutation isnecessary in order to drive sufficient current and thus power into thephase winding in order to maintain such speeds. However, the applicanthas identified that, once the motor is at this speed, improvements inthe efficiency of the motor may be achieved by retarding commutation.For a permanent-magnet motor, the torque-to-current ratio is at amaximum when the waveform of the phase current matches that of the backEMF. Improvements in the efficiency of the motor are therefore achievedby shaping the waveform of the phase current such that it better matchesthe waveform of the back EMF. In instances for which the phase currentrises faster than the back EMF at around zero-crossings in the back EMF,advanced commutation would cause the phase current to quickly lead theback EMF. By retarding commutation until after each zero-crossing in theback EMF, the rise in the phase current may be made to more closelyfollow that of the back EMF. As a result, the efficiency of the motormay be improved.

The method may comprise commutating the winding at times that areretarded relative to zero-crossings of back EMF when operating over aspeed range spanning at least 5 krpm, and more preferably at least 10krpm. As a result, improvements in the efficiency of the motor may beachieved over a relatively large range of speeds.

Changes in the magnitude of the supply voltage used to excite thewinding will influence the rate at which the phase current rises.Changes in the speed of the motor will influence the length of eachelectrical half-cycle and thus the rate at which the back EMF rises.Additionally, changes in the speed of the motor will influence themagnitude of the back EMF and thus the rate at which the phase currentrises. Accordingly, the method may comprise retarding commutation by aretard period and varying the retard period in response to changes inthe supply voltage and/or the speed of the motor. This then has theadvantage that the efficiency of the motor may be improved as the motoroperates over a range of supply voltages and/or motor speeds.Additionally, the amount of current and thus power that is driven intothe winding is sensitive to changes in the supply voltage and/or themotor speed. By varying the retard period in response to changes in thesupply voltage and/or motor speed, better control may be achieved overthe input or output power of the motor.

The method may comprise increasing the retard period in response to anincrease in the supply voltage and/or a decrease in the motor speed. Asthe magnitude of the supply voltage increases, the phase current risesat a faster rate. As the speed of the motor decreases, the back EMFrises at a slower rate. Additionally, the magnitude of the back EMFdecreases and thus the phase current rises at a faster rate. Byincreasing the retard period in response to an increase in the supplyvoltage and/or a decrease in the motor speed, the waveform of the phasecurrent may be made to better match that of the back EMF in response tochanges in the supply voltage and/or the motor speed. As a result, theefficiency of the motor may be improved when operating over a range ofsupply voltages and/or speeds.

The method may comprise dividing each electrical half-cycle into aconduction period followed by a freewheel period. The winding is thenexcited during the conduction period and freewheeled during thefreewheel period. Moreover, the method may comprise dividing theconduction period into a first excitation period, a further freewheelperiod and a second excitation period, and the winding may be excitedduring each excitation period and the winding may be freewheeled duringthe further freewheel period. Although commutation is retarded, thephase current may nevertheless rise at a faster rate than that of theback EMF. As a result, the phase current may eventually lead the backEMF. The secondary freewheel period serves to check momentarily the risein the phase current. Consequently, the phase current may be made tomore closely follow the rise of the back EMF during the conductionperiod, thereby improving the efficiency.

The method may comprise commutating the winding at times that areretarded relative to zero-crossings of back EMF when operating over afirst speed range, commutating the winding at times that are advancedrelative to zero-crossings of back EMF in the winding when operatingover a second speed range, and the second speed range is higher than thefirst speed range. When operating over the first speed range, the phasecurrent rises at a faster rate than that of the back EMF at aroundzero-crossings. Accordingly, by retarding commutation, improvements inthe efficiency of the motor may be achieved. When operating over thesecond speed range, the length of each electrical half-cycle is shorterand thus the back EMF rises at a faster rate. Additionally, themagnitude of the back EMF is higher and thus the phase current rises ata slower rate. The back EMF therefore rises at a faster rate but thephase current rises at a slower rate. As a result, the phase currentrises at a slower rate than that of the back EMF. Retarding commutationwould then only serve to worsen the efficiency of the motor. Moreover,it may not be possible to drive sufficient current and power into thewinding when operating over the second speed range if commutation isretarded. Accordingly, by employing retarded commutation over the firstspeed range and advanced commutation over the second speed range, theefficiency of the motor may be improved over both speed ranges.

The method may comprise dividing each electrical half-cycle of the motorinto a conduction period followed by a freewheel period when operatingover the first speed range and the second speed range, the winding beingexcited during the conduction period and freewheeled during thefreewheel period.

The method may comprise driving the motor at constant power (be it inputpower or output power) over the first speed range and the second speedrange. The power of the motor over the first speed range is then lowerthan that over the second speed range.

The method may comprise retarding commutation by a retard period whenthe motor operates over the first speed range, and advancing commutationby an advance period when the motor operates over the second speedrange. Constant power may then be achieved by varying the retard periodand the advance period in response to changes in the speed of the motor.Additionally or alternatively, constant power may be achieved by varyingthe length of the conduction period in response to changes in the speedof the motor.

The first speed range and the second speed range may each span at least5 krpm, and more preferably at least 10 krpm. As a result, improvementsin the efficiency of the motor may be achieved over relatively largespeed ranges.

The present invention also provides a control circuit configured toperform a method described in any one of the preceding paragraphs, aswell as a motor assembly that comprises the control circuit and abrushless permanent-magnet motor.

The control circuit may comprise an inverter for coupling to a windingof the motor, a gate driver module and a controller. The gate drivermodule then controls switches of the inverter in response to controlsignals received from the controller, and the controller generatescontrol signals to commutate the winding. More particularly, thecontroller generates control signals to commutate the winding at timesthat are retarded relative to zero-crossings of back EMF in the windingwhen the speed of the motor is greater than 50 krpm.

The present invention further provides a method of controlling abrushless permanent-magnet motor, the method comprising commutating awinding of the motor at times that are retarded relative tozero-crossings of back EMF in the winding, dividing each half of anelectrical cycle into a conduction period followed by a primaryfreewheel period, dividing the conduction period into a first excitationperiod, a secondary freewheel period, and a second excitation period,exciting a winding of the motor during each excitation period, andfreewheeling the winding during each freewheel period.

For a permanent-magnet motor, the torque-to-current ratio is at amaximum when the waveform of the phase current matches that of the backEMF Improvements in the efficiency of the motor are therefore achievedby shaping the waveform of the phase current such that it better matchesthe waveform of the back EMF. During excitation, the phase current mayrise faster than the back EMF at around the zero-crossings in the backEMF. Consequently, if the winding were commutated in advance of or insynchrony with the zero-crossings, the phase current would quickly leadthe back EMF. By retarding commutation until after each zero-crossing inthe back EMF, the rise in the phase current may be made to more closelyfollow that of the back EMF. As a result, the efficiency of the motormay be improved.

Although commutation is retarded, the phase current may neverthelessrise at a faster rate than that of the back EMF. As a result, the phasecurrent may eventually lead the back EMF. The secondary freewheel periodserves to check momentarily the rise in the phase current. Consequently,the phase current may be made to more closely follow the rise of theback EMF during the conduction period, thereby further improving theefficiency.

The secondary freewheel period may occur at a time when the back EMF inthe winding is rising, and the primary freewheel period may occur at atime when back EMF is principally falling. The primary freewheel periodmakes use of the inductance of the winding such that torque continues tobe generated by the phase current without any additional power beingdrawn from the power supply. As the back EMF falls, less torque isgenerated for a given phase current. Accordingly, by freewheeling thewinding during the period of falling back EMF, the efficiency of themotor may be improved without adversely affecting the torque.

The length of the secondary freewheel period may be less than each ofthe primary freewheel period, the first excitation period and the secondexcitation period. Consequently, the secondary freewheel period acts tocheck momentarily the rise of the phase current without adverselyaffecting the power of the motor.

The method may comprise exciting the winding with a supply voltage, andvarying the length of the conduction period in response to changes inthe supply voltage and/or the speed of the motor. As a result, bettercontrol may be achieved over the power of the motor.

As the supply voltage decreases, less current and thus less power aredriven into the motor over the same conduction period. Equally, as thespeed of the motor increases, the magnitude of the back EMF induced inthe winding increases. Less current and thus less power are then driveninto the motor over the same conduction period. Accordingly, in order tocompensate for this, the method may comprise increasing the conductionperiod in response to a decrease in the supply voltage and/or anincrease in the speed of the motor.

Changes in the magnitude of the supply voltage used to excite thewinding will influence the rate at which the phase current rises.Changes in the speed of the motor will influence the length of eachelectrical half-cycle and thus the rate at which the back EMF rises.Additionally, changes in the speed of the motor will influence themagnitude of the back EMF and thus the rate at which the phase currentrises. Accordingly, the method may comprise retarding commutation by aretard period, and varying the retard period in response to changes inthe supply voltage and/or the speed of the motor. This then has theadvantage that the efficiency of the motor may be improved as the motoroperates over a range of supply voltages and/or motor speeds.Additionally, the amount of current and thus power that is driven intothe winding is sensitive to changes in the supply voltage and/or themotor speed. By varying the retard period in response to changes in thesupply voltage and/or motor speed, better control may be achieved overthe input or output power of the motor.

The method may comprise increasing the retard period in response to anincrease in the supply voltage and/or a decrease in the motor speed. Asthe magnitude of the supply voltage increases, the phase current risesat a faster rate. As the speed of the motor decreases, the back EMFrises at a slower rate. Additionally, the magnitude of the back EMFdecreases and thus the phase current rises at a faster rate. Byincreasing the retard period in response to an increase in the supplyvoltage and/or a decrease in the motor speed, the waveform of the phasecurrent may be made to better match that of the back EMF in response tochanges in the supply voltage and/or the motor speed. As a result, theefficiency of the motor may be improved when operating over a range ofsupply voltages and/or speeds.

The method may comprise controlling the motor in the manner described inany one of the preceding paragraphs when operating at speeds greaterthan 50 krpm. At this relatively high speed, the length of eachelectrical half-cycle is relatively short and the magnitude of the backEMF is relatively large. Both of these factors would suggest thatadvanced commutation is necessary in order to drive sufficient currentand thus power into the phase winding in order to maintain such speeds.Indeed, it is likely that advanced commutation is necessary in order toaccelerate to such speeds. Nevertheless, the applicant has identifiedthat once at these speeds, commutation may be retarded so as to improvethe efficiency of the motor.

The method may comprise controlling the motor in the manner described inany one of the preceding paragraphs when operating over a speed rangethat spans at least 5 krpm, and more preferably at least 10 krpm. As aresult, improvements in the efficiency of the motor may be achieved overa relatively large speed range.

The present invention still further provides a control circuitconfigured to perform a method described in any one of the precedingparagraphs, as well as a motor assembly that comprises the controlcircuit and a brushless permanent-magnet motor.

The control circuit may comprise an inverter for coupling to a windingof the motor, a gate driver module and a controller. The gate drivermodule then controls switches of the inverter in response to controlsignals received from the controller. The controller generates controlsignals for commutating the winding at times that are retarded relativeto zero-crossings of back EMF in the winding. The controller is alsoresponsible for dividing each half of an electrical cycle into theconduction period and the primary freewheel period, and for dividing theconduction period into the first excitation period, the secondaryfreewheel period, and the second excitation period. The controller thengenerates control signals to excite the winding during each excitationperiod and to freewheel the winding during each freewheel period.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood, anembodiment of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a motor assembly in accordance with thepresent invention;

FIG. 2 is a schematic diagram of the motor assembly;

FIG. 3 details the allowed states of the inverter in response to controlsignals issued by the controller of the motor assembly;

FIG. 4 illustrates various waveforms of the motor assembly whenoperating in acceleration mode;

FIG. 5 illustrates various waveforms of the motor assembly whenoperating in high-power mode; and

FIG. 6 illustrates various waveforms of the motor assembly whenoperating in low-power mode.

DETAILED DESCRIPTION OF THE INVENTION

The motor assembly 1 of FIGS. 1 and 2 is powered by a DC power supply 2and comprises a brushless motor 3 and a control circuit 4.

The motor 3 comprises a four-pole permanent-magnet rotor 5 that rotatesrelative to a four-pole stator 6. Conductive wires wound about thestator 6 are coupled together to form a single phase winding 7.

The control circuit 4 comprises a filter 8, an inverter 9, a gate drivermodule 10, a current sensor 11, a voltage sensor 12, a position sensor13, and a controller 14.

The filter 8 comprises a link capacitor C1 that smoothes the relativelyhigh-frequency ripple that arises from switching of the inverter 9.

The inverter 9 comprises a full bridge of four power switches Q1-Q4 thatcouple the phase winding 7 to the voltage rails. Each of the switchesQ1-Q4 includes a freewheel diode.

The gate driver module 10 drives the opening and closing of the switchesQ1-Q4 in response to control signals received from the controller 14.

The current sensor 11 comprises a shunt resistor R1 located between theinverter and the zero-volt rail. The voltage across the current sensor11 provides a measure of the current in the phase winding 7 whenconnected to the power supply 2. The voltage across the current sensor11 is output to the controller 14 as signal, I_PHASE.

The voltage sensor 12 comprises a potential divider R2,R3 locatedbetween the DC voltage rail and the zero volt rail. The voltage sensoroutputs a signal, V_DC, to the controller 14 that represents ascaled-down measure of the supply voltage provided by the power supply2.

The position sensor 13 comprises a Hall-effect sensor located in a slotopening of the stator 6. The sensor 13 outputs a digital signal, HALL,that is logically high or low depending on the direction of magneticflux through the sensor 13. The HALL signal therefore provides a measureof the angular position of the rotor 5.

The controller 14 comprises a microcontroller having a processor, amemory device, and a plurality of peripherals (e.g. ADC, comparators,timers etc.). The memory device stores instructions for execution by theprocessor, as well as control parameters and lookup tables that areemployed by the processor during operation of the motor assembly 1. Thecontroller 14 is responsible for controlling the operation of the motor3 and generates four control signals S1-S4 for controlling each of thefour power switches Q1-Q4. The control signals are output to the gatedriver module 10, which in response drives the opening and closing ofthe switches Q1-Q4.

FIG. 3 summarises the allowed states of the switches Q1-Q4 in responseto the control signals S1-S4 output by the controller 14. Hereafter, theterms ‘set’ and ‘clear’ will be used to indicate that a signal has beenpulled logically high and low respectively. As can be seen from FIG. 3,the controller 14 sets S1 and S4, and clears S2 and S3 in order toexcite the phase winding 7 from left to right. Conversely, thecontroller 14 sets S2 and S3, and clears S1 and S4 in order to excitethe phase winding 7 from right to left. The controller 14 clears S1 andS3, and sets S2 and S4 in order to freewheel the phase winding 7.Freewheeling enables current in phase the winding 7 to re-circulatearound the low-side loop of the inverter 9. In the present embodiment,the power switches Q1-Q4 are capable of conducting in both directions.Accordingly, the controller 14 closes both low-side switches Q2,Q4during freewheeling such that current flows through the switches Q2,Q4rather than the less efficient diodes. Conceivably, the inverter 9 maycomprise power switches that conduct in a single direction only. In thisinstance, the controller 14 would clear S1, S2 and S3, and set S4 so asto freewheel the phase winding 7 from left to right. The controller 14would then clear S1, S3 and S4, and set S2 in order to freewheel thephase winding 7 from right to left. Current in the low-side loop of theinverter 9 then flows down through the closed low-side switch (e.g. Q4)and up through the diode of the open low-side switch (e.g. Q2).

The controller 14 operates in one of three modes: acceleration mode,low-power mode and high-power mode. Low-power mode and high-power modeare both steady-state modes. The controller 14 receives and periodicallymonitors a power-mode signal, POWER_MODE, in order to determine whichsteady-state mode should be employed. If the power-mode signal islogically low, the controller 14 selects low-power mode, and if thepower-mode signal is logically high, the controller 14 selectshigh-power mode. When operating in low-power mode, the controller 14drives the motor 3 over an operating speed range of 60-70 krpm. Whenoperating in high-power mode, the controller 14 drives the motor 3 overan operating speed range of 90-100 krpm. Acceleration mode is then usedto accelerate the motor 3 from stationary to the lower limit of eachoperating speed range.

In all three modes the controller 14 commutates the phase winding 7 inresponse to edges of the HALL signal. Each HALL edge corresponds to achange in the polarity of the rotor 5, and thus a change in the polarityof the back EMF induced in the phase winding 7. More particularly, eachHALL edge corresponds to a zero-crossing in the back EMF. Commutationinvolves reversing the direction of current through the phase winding 7.Consequently, if current is flowing through the phase winding 7 in adirection from left to right, commutation involves exiting the windingfrom right to left.

In the discussion below, reference is made frequently to the speed ofthe motor 3. The speed of the motor 3 is determined from the intervalbetween successive edges of the HALL signal, which will hereafter bereferred to as the HALL period.

Acceleration Mode

At speeds below 20 krpm, the controller 14 commutates the phase winding7 in synchrony with each HALL edge. At speeds at or above 20 krpm, thecontroller 14 commutates the phase winding 7 in advance of each HALLedge. In order to commutate the phase winding 7 in advance of aparticular HALL edge, the controller 14 acts in response to thepreceding HALL edge. In response to the preceding HALL edge, thecontroller 14 subtracts an advance period, T_ADV, from the HALL period,T_HALL, in order to obtain a commutation period, T_COM:

T_COM=T_HALL−T_ADV

The controller 14 then commutates the phase winding 7 at a time, T_COM,after the preceding HALL edge. As a result, the controller 14 commutatesthe phase winding 7 in advance of the subsequent HALL edge by theadvance period, T_ADV.

Irrespective of whether commutation in synchronous or advanced, thecontroller 14 sequentially excites and freewheels the phase winding 7over each half of an electrical cycle when operating in accelerationmode. More particularly, the controller 14 excites the phase winding 7,monitors the current signal, I_PHASE, and freewheels the phase winding 7when the current in the phase winding 7 exceeds a predefined limit.Freewheeling then continues for a predefined freewheel period duringwhich time current in the phase winding 7 falls to a level below thecurrent limit. At the end of the freewheel period the controller 14again excites the phase winding 7. This process of exciting andfreewheeling the phase winding 7 continues over the full length of theelectrical half-cycle. The controller 14 therefore switches fromexcitation to freewheeling multiple times during each electricalhalf-cycle.

FIG. 4 illustrates the waveforms of the HALL signal, the back EMF, thephase current, the phase voltage, and the control signals over a coupleof HALL periods when operating in acceleration mode. In FIG. 4 the phasewinding 7 is commutated in synchrony with the HALL edges.

At relatively low speeds, the magnitude of the back EMF induced in thephase winding 7 is relatively small. Current in the phase winding 7therefore rises relatively quickly during excitation, and fallsrelatively slowly during freewheeling. Additionally, the length of eachHALL period and thus the length of each electrical half-cycle isrelatively long. Consequently, the frequency at which the controller 14switches from excitation to freewheeling is relatively high. However, asthe rotor speed increases, the magnitude of the back EMF increases andthus current rises at a slower rate during excitation and falls at aquicker rate during freewheeling. Additionally, the length of eachelectrical half-cycle decreases. As a result, the frequency of switchingdecreases.

The controller 14 continues to operate in acceleration mode until thespeed of the rotor 5 reaches the lower limit of the operating speedrange of the selected power mode. So, for example, if high-power mode isselected, the controller 14 continues to operate in acceleration modeuntil the speed of the rotor 5 reaches 90 krpm.

High-Power Mode

The controller 14 commutates the phase winding in advance of each HALLedge. Advanced commutation is achieved in the same manner as thatdescribed above for acceleration mode.

When operating in high-power mode, the controller 14 divides each halfof an electrical cycle into a conduction period followed by a freewheelperiod. The controller 14 then excites the phase winding 7 during theconduction period and freewheels the phase winding 7 during thefreewheel period. The phase current is not expected to exceed thecurrent limit during excitation. Consequently, the controller 14switches from excitation to freewheeling only once during eachelectrical half-cycle.

The controller 14 excites the phase winding 7 for a conduction period,T_CD. At the end of the conduction period, the controller 14 freewheelsthe phase winding 7. Freewheeling then continues indefinitely until suchtime as the controller 14 commutates the phase winding 7. The controller14 therefore controls operation of the motor 3 using two parameters: theadvance period, T_ADV, and the conduction period, T_CD.

FIG. 5 illustrates the waveforms of the HALL signal, the back EMF, thephase current, the phase voltage, and the control signals over a coupleof HALL periods when operating in high-power mode.

The magnitude of the supply voltage used to excite the phase winding 7may vary. For example, the power supply 2 may comprise a battery thatdischarges with use. Alternatively, the power supply 2 may comprise anAC source, rectifier and smoothing capacitor that provide a relativelysmooth voltage, but the RMS voltage of the AC source may vary. Changesin the magnitude of the supply voltage will influence the amount ofcurrent that is driven into the phase winding 7 during the conductionperiod. As a result, the power of the motor 3 is sensitive to changes inthe supply voltage. In addition to the supply voltage, the power of themotor 3 is sensitive to changes in the speed of the rotor 5. As thespeed of the rotor 5 varies (e.g. in response to changes in load), sotoo does the magnitude of the back EMF. Consequently, the amount ofcurrent driven into the phase winding 7 during the conduction period mayvary. The controller 14 therefore varies the advance period and theconduction period in response to changes in the magnitude of the supplyvoltage. The controller 14 also varies the advance period in response tochanges in the speed of the rotor 5.

The controller 14 stores a voltage lookup table that comprises anadvance period, T_ADV, and a conduction period, T_CD, for each of aplurality of different supply voltages. The controller 14 also stores aspeed lookup table that comprises a speed-compensation value for each ofa plurality of different rotor speeds and different supply voltages. Thelookup tables store values that achieve a particular input or outputpower at each voltage and speed point. In the present embodiment, thelookup tables store values that achieve constant output power for themotor 3 over a range of supply voltages as well as over the operatingspeed range for high-power mode.

The V_DC signal output by the voltage sensor 12 provides a measure ofthe supply voltage, whilst the length of the HALL period provides ameasure of the rotor speed. The controller 14 indexes the voltage lookuptable using the supply voltage to select a phase period and a conductionperiod. The controller 14 then indexes the speed lookup table using therotor speed and the supply voltage to select a speed-compensation value.The controller 14 then adds the selected speed-compensation value to theselected phase period so as to obtain a speed-compensated phase period.The commutation period, T_COM, is then obtained by subtracting thespeed-compensated phase period from the HALL period, T_HALL.

The speed lookup table stores speed-compensation values that depend notonly on the speed of the rotor 5 but also on the magnitude of the supplyvoltage. The reason for this is that, as the supply voltage decreases, aparticular speed-compensation value has a smaller net effect on theoutput power of the motor 3. By storing speed-compensation values thatdepend on both the rotor speed and the supply voltage, better controlover the output power of the motor 3 may be achieved in response tochanges in the rotor speed.

It will be noted that two lookup tables are used to determine theadvance period. The first lookup table (i.e. the voltage lookup table)is indexed using the supply voltage. The second lookup table (i.e. thespeed lookup table) is indexed using both the rotor speed and the supplyvoltage. Since the second lookup table is indexed using both rotor speedand supply voltage, one might question the need for two lookup tables.However, the advantage of using two lookup tables is that differentvoltage resolutions may be used. The output power of the motor 3 isrelatively sensitive to the magnitude of the supply voltage. Incontrast, the effect that the speed-compensation value has on the outputpower is less sensitive to the supply voltage. Accordingly, by employingtwo lookup tables, a finer voltage resolution may be used for thevoltage lookup table, and coarser voltage resolution may be used for thespeed lookup table. As a result, relatively good control over the outputpower of the motor 3 may be achieved through the use of smaller lookuptables, which then reduces the memory requirements of the controller 14.

Low-Power Mode

The controller 14 commutates the phase winding 7 at times that areretarded relative to the HALL edges. Retarded commutation is achieved ina manner similar to that for advanced commutation. In response to a HALLedge, the controller 14 adds a retard period, T_RET, to the HALL period,T_HALL, in order to obtain a commutation period, T_COM:

T_COM=T_HALL+T_RET

The controller 14 then commutates the phase winding 7 at a time, T_COM,after the HALL edge. As a result, the controller 14 commutates the phasewinding 7 at a time T_RET after the subsequent HALL edge.

When operating in low-power mode, the controller 14 divides each half ofan electrical cycle into a conduction period followed by a primaryfreewheel period. The controller 14 then divides the conduction periodinto a first excitation period, followed by a secondary freewheelperiod, followed by a second excitation period. The controller 14 thenexcites the phase winding 7 during each of the two excitation periodsand freewheels the phase winding 7 during each of the two freewheelperiods. As in high-power mode, the phase current is not expected toexceed the current limit during excitation. Accordingly, the controller14 switches from excitation to freewheeling twice during each electricalhalf-cycle.

FIG. 6 illustrates the waveforms of the HALL signal, the back EMF, thephase current, the phase voltage, and the control signals over a coupleof HALL periods when operating in low-power mode.

As in high-power mode, the controller 14 varies the retard period andthe conduction period in response to changes in the magnitude of thesupply voltage, and the controller 14 varies the retard period inresponse to changes in the speed of the rotor 5. The controller 14therefore stores a further voltage lookup table that comprises differentretard periods, T_RET, and different excitation periods, T_EXC, fordifferent supply voltages. The controller 14 also stores a further speedlookup table that comprises speed-compensation values for differentrotor speeds and different supply voltages. The lookup tables employedin low-power mode therefore differ from those employed in high-powermode only in that tables store retard periods rather than advanceperiods, and excitation periods rather than conduction periods. As inhigh-power mode, the lookup tables employed in low-power mode storevalues that achieve constant output power for the motor 3 over the samerange of supply voltages and over the operating speed range forlow-power mode.

During operation, the controller 14 indexes the voltage lookup tableusing the supply voltage to select a retard period and an excitationperiod. The selected excitation period is then used to define both thefirst excitation period and the second excitation period, i.e. duringthe conduction period the controller 14 excites the phase winding 7 forthe selected excitation period, freewheels the phase winding 7 for thesecondary freewheel period, and excites the phase winding 7 again forthe selected excitation period. As a result, the secondary freewheelperiod occurs at the centre of the conduction period.

In comparison to high-power mode, the excitation of the phase winding 7differs in two important ways. First, the controller 14 retardscommutation. Second, the controller 14 introduces a secondary freewheelperiod into the conduction period. The reasons for and benefits of thesetwo differences will now be explained.

When operating in high-power mode, advanced commutation is necessary inorder to achieve the necessary output power. As the speed of the rotor 5increases, the HALL period decreases and thus the time constant (L/R)associated with the phase inductance becomes increasingly important.Additionally, the back EMF induced in the phase winding 7 increases,which in turn influences the rate at which phase current rises. Ittherefore becomes increasingly difficult to drive current and thus powerinto the phase winding 7. By commutating the phase winding 7 in advanceof each HALL edge, and thus in advance of the zero-crossings in the backEMF, the supply voltage is boosted momentarily by the back EMF. As aresult, the direction of current through the phase winding 7 is morequickly reversed. Additionally, the phase current is caused to lead theback EMF, which helps to compensate for the slower rate of current rise.Although this then generates a short period of negative torque, this isnormally more than compensated by the subsequent gain in positivetorque.

When operating in low-power mode, the length of the HALL period islonger and thus the back EMF rises at a slower rate. Additionally, themagnitude of the back EMF is lower and thus current in the phase winding7 rises at a faster rate for a given supply voltage. The back EMFtherefore rises at a slower rate but the phase current rises at a fasterrate. It is not therefore necessary to commutate the phase winding 7 inadvance of the HALL edges in order to achieve the desired output power.Moreover, for reasons that will now be explained, the efficiency of themotor assembly 3 is improved by retarding commutation.

During excitation, the torque-to-current ratio is at a maximum when thewaveform of the phase current matches that of the back EMF Improvementsin the efficiency of the motor 3 are therefore achieved by shaping thewaveform of the phase current such that it better matches the waveformof the back EMF, i.e. by reducing the harmonic content of the phasecurrent waveform relative to the back EMF waveform. As noted in thepreceding paragraph, when operating in low-power mode, the back EMFrises at a slower rate but the phase current rises at a faster rate.Indeed, when operating in low-power mode, the phase current rises fasterthan the back EMF when the magnitude of the back EMF is relatively low,i.e. at around zero-crossings in the back EMF. Consequently, if thephase winding 7 were commutated in advance of or in synchrony with theHALL edges, the phase current would quickly lead the back EMF. Inhigh-power mode, it was necessary for the phase current to initiallylead the back EMF in order to compensate for the shorter HALL period andthe slower rise in the phase current. In low-power mode, however, it isnot necessary for the phase current to lead the back EMF in order toachieve the necessary output power. By retarding commutation until aftereach HALL edge, the phase current more closely follows the rise in theback EMF. As a result, the efficiency of the motor assembly 1 isimproved.

The secondary freewheel period acts to further improve the efficiency ofthe motor 3. As a result of retarding commutation, the phase currentmore closely matches that of the back EMF. Nevertheless, the phasecurrent continues to rise at a faster rate than that of the back EMF.Consequently, the phase current eventually overtakes the back EMF. Byintroducing a relatively small secondary freewheel period into theconduction period, the rise in the phase current is checked momentarilysuch that the rise in the phase current more closely follows the rise inthe back EMF. As a result, the harmonic content of the phase currentwaveform relative to the back EMF waveform is further reduced and thusthe efficiency of the motor 3 is further increased.

Acceleration mode is used to accelerate the motor 3 from stationary tothe lower limit of each operating speed range. Consequently, thecontroller 14 operates in acceleration mode between 0 and 90 krpm whenhigh-power mode is selected, and between 0 and 60 krpm when low-powermode is selected. Irrespective of which power mode has been selected,the controller 14 commutates the phase winding 7 in synchrony with theHALL edges between 0 and 20 krpm. The controller 14 then commutates thephase winding 7 in advance of the HALL edges as the motor 3 acceleratesfrom 20 to 90 krpm (high-power mode) or from 20 to 60 krpm (low-powermode). When operating in acceleration mode, the controller 14 employs anadvance period that remains fixed as the rotor 5 accelerates.Irrespective of whether high-power mode or low-power mode is selected,the controller 14 indexes the voltage lookup table employed inhigh-power mode using the supply voltage in order to select an advanceperiod. The selected advance period is then used by the controller 14during acceleration mode. Conceivably, the efficiency of the motor 3 maybe improved by employing an advance period that varies with rotor speed.However, this would then require an additional lookup table. Moreover,acceleration mode is typically short-lived, with the controller 14operating predominantly in low-power mode or high-power mode.Consequently, any efficiency improvements that may be made by varyingthe advance period within acceleration mode are unlikely to contributesignificantly to the overall efficiency of the motor 3.

On switching from acceleration mode to low-power mode, the controller 14switches from advanced commutation to retarded commutation. Whenoperating within the low-power mode, the controller 14 is able to drivesufficient current and thus power into the phase winding 7 during eachelectrical half-cycle whilst simultaneously employing retardedcommutation in order to improve the efficiency of the motor assembly 1.In contrast, during acceleration, advanced commutation is necessary inorder to ensure that sufficient current and thus power is driven intothe phase winding 7 during each electrical half-cycle. If the controller14 were to retard or synchronise commutation during acceleration, therotor 5 would fail to accelerate to the required speed. Accordingly,whilst retarded commutation may be employed in order to maintain therotor speed over a speed range of 60-70 krpm, advanced commutation isnecessary in order to ensure that the rotor accelerates to 60 krpm.

Whilst it is known to retard commutation when operating at relativelylow speeds, it is completely unknown to retard commutation whenoperating at relatively high speeds, i.e. at speeds in excess of 50krpm. At these relatively high speeds, the relatively short length ofeach HALL period and the magnitude of the back EMF would suggest thatadvanced commutation is necessary. Indeed, advanced commutation isnecessary in order to accelerate the motor to such speeds. The applicanthas, however, identified that once at these speeds, commutation may beretarded so as to improve the efficiency of the motor 3.

In the embodiment described above, the controller 14 employs twosteady-state modes: high-power mode and low-power mode. In high-powermode, the controller 14 commutates the phase winding 7 in advance ofzero-crossings in the back EMF. In low-power mode, the controller 14commutates the phase winding 7 in retard of zero-crossings in the backEMF. The advance period and the retard period may each be regarded as aphase period, T_PHASE, and the commutation period, T_COM may be definedas:

T_COM=T_HALL−T_PHASE

If the phase period is positive, commutation occurs before the HALL edge(i.e. advanced commutation), and if the phase period is negative,commutation occurs after the HALL edge (i.e. retarded commutation).Employing the same scheme to commutate the phase winding 7 in bothhigh-power mode and low-power mode simplifies the control. However,conceivably different methods may be used to commutate the phase winding7. For example, when operating in low power mode, the controller 14 maysimply commutate the phase winding 7 at a time, T_RET, after each HALLedge.

In the embodiment described above, the controller 14 varies only thephase period (i.e. the advance period in high-power mode and the retardperiod in low-power mode) in response to changes in the rotor speed. Incomparison to the conduction period, the input power of the motor 3 istypically more sensitive to changes in the phase period. Accordingly,better control over the output power of the motor 3 may be achieved byvarying the phase period. Nevertheless, in spite of these advantages,the controller 14 may instead vary only the conduction period inresponse to changes in the rotor speed. Alternatively, the controller 14may vary both the phase period and the conduction period in response tochanges in the rotor speed. This may be necessary if, for example, theoutput power of the motor 3 cannot be controlled adequately by varyingjust the phase period. Alternatively, improvements in the efficiency ofthe motor 3 may be achieved by varying both the phase period and theconduction period in response to changes in the rotor speed. However, adisadvantage of varying both the phase period and the conduction periodis that additional lookup tables are required, thus placing additionaldemands on the memory of the controller 14.

In the embodiment described above, the controller 14 varies the phaseperiod and the conduction period in response to changes in the supplyvoltage. This then has the advantage that the efficiency of the motor 3may be better optimised at each voltage point. Nevertheless, it may bepossible to achieve the desired control over the output power of themotor 3 by varying just one of the phase period and the conductionperiod. Since the output power of the motor 3 is more sensitive tochanges in the phase period, better control over the output power of themotor 3 may be achieved by varying the phase period.

The controller 14 may therefore be said to vary the phase period and/orthe conduction period in response to changes in the supply voltage andthe rotor speed. Whilst the two periods may be varied in response tochanges in the supply voltage and the rotor speed, the controller 14could conceivably vary the periods in response to only one of the supplyvoltage and the rotor speed. For example, the voltage provided by thepower supply 2 may be relatively stable. In the instance, the controller14 might vary the phase period and/or the conduction period in responseto changes in the rotor speed only. Alternatively the motor 3 may berequired to operate at constant speed or over a relatively small rangeof speeds within low-power mode and high-power mode. In this instance,the controller 14 might vary the phase period and/or the conductionperiod in response to changes in the supply voltage only. Accordingly,in a more general sense, the controller 14 may be said to vary the phaseperiod and/or the conduction period in response to changes in the supplyvoltage and/or the rotor speed. Moreover, rather than storing a voltagelookup table or a speed lookup table, the controller 14 may be said tostore a power lookup table that comprises different control values fordifferent supply voltages and/or rotor speeds. Each control value thenachieves a particular output power at each voltage and/or speed point.The controller 14 then indexes the power lookup table using the supplyvoltage and/or the rotor speed to select a control value from the powerlookup table. The control value is then used to define the phase periodor the conduction period.

In the embodiment described above, the controller 14 stores lookuptables that comprises conduction periods for use in high-power mode andexcitation periods for use in low-power mode. However, the same level ofcontrol may be achieved by different means. For example, rather thanstoring a lookup table of conduction periods and excitation periods, thecontroller 14 could store a lookup table of primary freewheel periods,which is likewise indexed using the magnitude of the supply voltageand/or the speed of the rotor 5. The conduction period would then beobtained by subtracting the primary freewheel period from the HALLperiod, and each excitation period would be obtained by subtracting theprimary and the secondary freewheel periods from the HALL period anddividing the result by two:

T_CD=T_HALL−T_FW_(—)1

T_EXC=(T_HALL−T_FW_(—)1−T_FW_(—)2)/2

where T_CD is the conduction period, T_EXC is each of the first andsecond excitation periods, T_HALL is the HALL period, T_FW_(—)1 is theprimary freewheel period, and T_FW_(—)2 is the secondary freewheelperiod.

In the embodiment described above, the secondary freewheel period occursat the very centre of the conduction period. This is achieved byensuring that the same excitation period is used to define the lengthsof the first excitation period and the second excitation period. Thereare at least two advantages in ensuring that the secondary freewheelperiod occurs at the centre of the conduction period. First, theharmonic content of the phase current is better balanced over the twoexcitation periods. As a result, the total harmonic content of the phasecurrent over the conduction period is likely to be lower than if the twoexcitation periods were of different lengths. Second, the lookup tableneed only store one excitation period for each voltage point. As aresult, less memory is required for the lookup table. In spite of theaforementioned advantages, it may be desirable to alter the position ofthe secondary freewheel period in response to changes in the supplyvoltage and/or the rotor speed. This may be achieved by employing alookup table that stores a first excitation period and a secondexcitation period for different voltages and/or speeds.

The controller 14 employs a secondary freewheel period that is fixed inlength. This then has the advantage of reducing the memory requirementsof the controller 14. Alternatively, however, the controller 14 mightemploy a secondary freewheel period that varies in response to changesin the supply voltage and/or the rotor speed. In particular, thecontroller 14 may employ a secondary freewheel period that increases inresponse to an increase in the supply voltage or a decrease in the rotorspeed. As the supply voltage increases, current in the phase winding 7rises at a faster rate during excitation, assuming that the rotor speedand thus the magnitude of the back EMF is unchanged. As a result, theharmonic content of the phase current waveform relative to the back EMFwaveform is likely to increase. By increasing the length of thesecondary freewheel period in response to an increase in the supplyvoltage, the rise in the phase current is checked for a longer periodand thus the harmonic content of the phase current waveform may bereduced. As the rotor speed decreases, the length of the HALL periodincreases and thus the back EMF rises at a slower rate. Additionally,the magnitude of the back EMF decreases and thus current in the phasewinding 7 rises at a faster rate, assuming that the supply voltage isunchanged. Consequently, as the rotor speed decreases, the back EMFrises at a slower rate but the phase current rises at a faster rate. Theharmonic content of the phase current waveform relative to the back EMFwaveform is therefore likely to increase. By increasing the secondaryfreewheel period in response to a decrease in the rotor speed, the risein the phase current is checked for a longer period and thus theharmonic content of the phase current waveform may be reduced.Accordingly, increasing the secondary freewheel period in response to anincrease in the supply voltage and/or a decrease in the rotor speed mayresult in further improvements in efficiency.

The length of the secondary freewheel period is relatively short and isintended only to check momentarily the rise in the phase current.Accordingly, the secondary freewheel period is shorter than both theprimary freewheel period and each of the excitation periods. The actuallength of the secondary freewheel period will depend upon the particularcharacteristics of the motor assembly 1, e.g. the inductance of thephase winding 7, the magnitude of the supply voltage, the magnitude ofthe back EMF etc. Irrespective of the length, the secondary freewheelperiod occurs during a period of rising back EMF in the phase winding 7.This is contrast to the primary freewheel period, which occursprincipally if not wholly during a period of falling back EMF. Theprimary freewheel period makes use of the inductance of the phasewinding 7 such that torque continues to be generated by the phasecurrent without any additional power being drawn from the power supply2. As the back EMF falls, less torque is generated for a given phasecurrent. Accordingly, by freewheeling the phase winding 7 during theperiod of falling back EMF, the efficiency of the motor assembly 1 maybe improved without adversely affecting the torque.

1. A method of controlling a brushless permanent-magnet motor, themethod comprising commutating a winding of the motor at times that areretarded relative to zero-crossings of back EMF in the winding whenoperating at speeds greater than 50 krpm.
 2. The method of claim 1,wherein the method comprises commutating the winding at times that areretarded relative to zero-crossings of back EMF when operating over aspeed range spanning at least 5 krpm.
 3. The method of claim 1, whereinthe method comprising commutating the winding at times that are retardedrelative to zero-crossings of back EMF in the winding by a retardperiod, and varying the retard period in response to changes in a supplyvoltage used to excite the winding or the speed of the motor.
 4. Themethod of claim 3, wherein the method comprises increasing the retardperiod in response to increase in the supply voltage or a decrease inthe speed of the motor.
 5. The method of claim 1, wherein the methodcomprises dividing each electrical half-cycle into a conduction periodfollowed by a freewheel period, the winding being excited during theconduction period and freewheeled during the freewheel period.
 6. Themethod of claim 5, wherein the method comprises dividing the conductionperiod into a first excitation period, a further freewheel period and asecond excitation period, the winding is excited during each excitationperiod and the winding is freewheeled during the further freewheelperiod.
 7. The method of claim 1, wherein the method comprisescommutating the winding at times that are retarded relative tozero-crossings of back EMF when operating over a first speed range,commutating the winding at times that are advanced relative tozero-crossings of back EMF in the winding when operating over a secondspeed range, and the second speed range is higher than the first speedrange.
 8. The method of claim 7, wherein the method comprises dividingeach electrical half-cycle of the motor into a conduction periodfollowed by a freewheel period when operating over the first speed rangeand the second speed range, the winding being excited during theconduction period and freewheeled during the freewheel period.
 9. Themethod of claim 7, wherein the method comprises driving the motor atconstant power over the first speed range and driving the motor at ahigher constant power over the second speed range.
 10. The method ofclaim 7, wherein the first speed range and the second speed range eachspan at least 5 krpm.
 11. A control circuit for a brushlesspermanent-magnet motor, the control circuit being configured tocommutate a winding of the motor at times that are retarded relative tozero-crossings of back EMF in the winding when operating at speedsgreater than 50 krpm.
 12. The control circuit of claim 11, wherein thecontrol circuit is included in a motor assembly for a brushlesspermanent-magnet motor.
 13. A method of controlling a brushlesspermanent-magnet motor, the method comprising: commutating a winding ofthe motor at times that are retarded relative to zero-crossings of backEMF in the winding; dividing each half of an electrical cycle into aconduction period followed by a primary freewheel period; dividing theconduction period into a first excitation period, a secondary freewheelperiod, and a second excitation period; exciting a winding of the motorduring each excitation period; and freewheeling the winding during eachfreewheel period.
 14. The method of claim 13, wherein the secondaryfreewheel period occurs at a time when back EMF in the winding isrising, and the primary freewheel period occurs at a time when back EMFis principally falling.
 15. The method of claim 13, wherein the lengthof the secondary freewheel period is less than each of the primaryfreewheel period, the first excitation period and the second excitationperiod.
 16. The method of claim 13, wherein the method comprisesexciting the winding with a supply voltage, and varying the length ofthe conduction period in response to changes in the supply voltage orthe speed of the motor.
 17. The method of claim 16, wherein the methodcomprises increasing the length of the conduction period in response toa decrease in the supply voltage or an increase in the speed of themotor.
 18. The method of claim 13, wherein the method comprisingcommutating the winding at times that are retarded relative tozero-crossings of back EMF in the winding by a retard period, excitingthe winding with a supply voltage, and varying the length of the retardperiod in response to changes in the supply voltage or the speed of themotor.
 19. The method of claim 18, wherein the method comprisesincreasing the retard period in response to increase in the supplyvoltage or a decrease in the speed of the motor.
 20. The method of claim13, wherein the method comprises commutating the winding at times thatare retarded relative to zero-crossings of back EMF, dividing each halfof an electrical cycle into the conduction period and primary freewheelperiod, and dividing the conduction period into the first excitationperiod, the secondary freewheel period, and the second excitation periodwhen operating at speeds greater than 50 krpm.
 21. The method of claim13, wherein the method comprises commutating the winding at times thatare retarded relative to zero-crossings of back EMF, dividing each halfof an electrical cycle into the conduction period and primary freewheelperiod, and dividing the conduction period into the first excitationperiod, the secondary freewheel period, and the second excitation periodwhen operating over a speed range that spans at least 5 krpm.
 22. Themethod of claim 21, wherein the method comprises driving the motor atconstant power over the speed range.
 23. A control circuit for abrushless permanent-magnet motor, the control circuit being configuredto: commutate a winding of the motor at times that are retarded relativeto zero-crossings of back EMF in the winding; divide each half of anelectrical cycle into a conduction period followed by a primaryfreewheel period; divide the conduction period into a first excitationperiod, a secondary freewheel period, and a second excitation period;excite a winding of the motor during each excitation period; andfreewheel the winding during each freewheel period.
 24. The controlcircuit of claim 23, wherein the control circuit is included in a motorassembly for a brushless permanent-magnet motor.